Thin film integrated circuits (ICs), commonly called “chips” or “IC chips”, are fabricated by a photolithographic process by which a photolithographic mask is positioned adjacent a substrate, such as a wafer, for adding features to or subtracting features from the substrate. Thus, each mask defines a layer of material to be added to, or removed from, the substrate. Typically, the mask is used with another material, such as a photoresist, which is exposed by light through the photographic mask to define the feature being formed on the substrate to form the chip. The photoresist is then developed to define the feature, and the feature is then deposited onto, or etched from, a layer on the substrate forming the chip.
The density of features on a chip is, in part, limited by the size of the photographic image on the mask, as well as the ability of the photoresist to accurately reproduce the feature defined by the photographic image. However, light waves through the mask are often distorted through the mask when patterning the photoresist. Moreover, the photoresist itself may diffuse during the photolithographic process, thereby inaccurately reproducing the features on the mask. Hence, either or both light diffraction and photoresist diffusion adversely affects the resulting shapes of features on the substrate.
A technique known as optical proximity correction (OPC) permits compensation for distortion due to diffraction of light waves by altering the photographic image into a shape so that it will form the correct shape of the feature on the substrate. Using OPC, the features formed on the substrate may be smaller and more closely packed (dense) without forming breaks or shorts in the circuitry.
However, OPC techniques are time consuming and expensive, particularly for masks for fabricating chips with hundreds of thousands, or even millions, of gates. In such circumstances OPC can only be applied to local regions of the mask; it is not possible to apply OPC to all areas of a mask simultaneously. Consequently, it has been necessary to apply OPC techniques to multiple regions of the mask independently, and resolve conflicts across region bounds separately. Moreover, application of OPC became repetitive where the chip includes numerous copies of the same or similar circuit.
Accordingly, there is a need for an improved technique to form shapes on a mask that, when light diffracts through the mask, accurately reproduces the desired shape for the feature on the substrate. There is also a need for an improved technique that permits assigning local images of the mask to classes with other similar local images, and performing OPC on the class to derive mask correction for all local images of the class. There is also a need for a reliable technique to identify photoresist diffusion to correct mask boundaries.